Next Article in Journal
A Review of Materials for Carbon Dioxide Capture
Next Article in Special Issue
Pd Catalysts Based on C3N4-Modified Activated Carbon Derived from Biomass Residues for the Dehydrogenation of Formic Acid
Previous Article in Journal
Zinc Indium Sulfide Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 3
10.3390/catal15030272
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Butanol Production by Ethanol Condensation: Improvements and Limitations in the Rational Design of Cu-Ni-MgO/Graphite Catalysts

by
Inmaculada Rodríguez-Ramos
1,
Cristina Lopez-Olmos
1 and
Antonio Guerrero-Ruiz
2,3,*
1
Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049 Madrid, Spain
2
Departamento Química Inorgánica y Técnica, Facultad de Ciencias UNED, Edificio Las Rozas 1, Av. de Esparta s/n, Las Rozas, 28232 Madrid, Spain
3
Grupo de Diseño y Aplicación de Catalizadores Heterogéneos, UNED, Unidad Asociada al CSIC por el ICP, 28049 Madrid, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(3), 272; https://doi.org/10.3390/catal15030272
Submission received: 31 January 2025 / Revised: 10 March 2025 / Accepted: 12 March 2025 / Published: 13 March 2025
(This article belongs to the Special Issue Carbon-Based Catalysts to Address Environmental Challenges)
Browse Figures
Versions Notes

Abstract

:
The advancement in catalytic processes utilizing sustainable raw materials, such as bioethanol, represents a key scientific challenge in this century. One potential approach to producing 1-butanol, a compound primarily obtained from petroleum-derived sources, is through the Guerbet reaction. For this transformation, various multifunctional catalysts have been explored, some of which incorporate Cu and/or Ni nanoparticles that facilitate hydrogenation and dehydrogenation reactions, along with magnesium oxide, which provides the necessary acid/base functionality. To promote nanoparticle formation and maximize the exposed active surface area, high-surface-area graphite (HSAG), a hydrophobic and inert support material, emerges as a promising candidate. In this study, different catalyst formulations containing these components were tested under moderate reaction conditions, at temperatures between 440 and 580 K and a pressure of 50 bar. A strong correlation was observed between butanol selectivity and the presence of medium–high strength basic sites, complemented by moderate acidity. Furthermore, optimizing the copper and nickel loadings to 4 wt.% Cu and 1 wt.% Ni significantly minimized the formation of unwanted byproducts. The highest butanol selectivity (44%) was achieved using a 4Cu1Ni-Mg/HSAG catalyst, which had been pretreated in helium at 723 K before H2 reduction, yielding approximately 9% 1-butanol.

Graphical Abstract

1. Introduction

The Guerbet reaction for butanol synthesis from bioethanol has gained increasing attention over the years [1,2,3], as butanol is regarded as a promising fuel additive. One of its key advantages is its similarity to gasoline, particularly in energy density and lower corrosiveness compared to bioethanol, which results from its reduced water affinity. This property allows butanol to be compatible with existing combustion engine technologies [4,5]. Given these characteristics, butanol derived from biomass has been explored as a potential precursor for the production of aviation fuels [6].
The conversion of bioethanol to 1-butanol via the Guerbet reaction follows multiple reaction steps (see Scheme 1), which can be outlined as follows: (R1) ethanol dehydrogenation to form acetaldehyde, (R2) aldol condensation of acetaldehyde, yielding 3-hydroxybutanal, which can subsequently undergo dehydration to form 2-butenal (R3), followed by sequential hydrogenation reactions (R4 and R5) leading to butyraldehyde and ultimately 1-butanol [5,7]. Since this is a multi-step transformation, several side reactions may occur. According to previous studies, common by-products include 1,1-diethoxyethane, formed through acid-catalyzed acetylation of acetaldehyde with ethanol [8], and ethyl acetate, resulting from the dehydrogenation of a hemiacetal intermediate [3]. Additionally, diethyl ether and ethylene can be produced via acid-mediated ethanol dehydration [3,9]. At temperatures above 500 K, 1-butanol may undergo further condensation, forming higher alcohols such as 1-hexanol and 1-octanol [10]. Other reported by-products include methane, carbon monoxide, acetone, propane, pentane, butane, and butenes [9,11,12]. Ultimately, the selection of an appropriate catalyst plays a crucial role in steering the reaction pathway toward higher 1-butanol yields, minimizing undesired side reactions.
Since aldol condensation relies on the presence of basic sites, while the dehydration of 3-hydroxybutanal requires acidic sites, an effective catalyst must incorporate both acidic and basic surface functionalities, along with metallic components to facilitate dehydrogenation/hydrogenation steps [3,13]. Traditionally, the industrial Guerbet process has been carried out using homogeneous catalysts, primarily alkali metal hydroxides and transition metal complexes. However, heterogeneous catalysts offer key advantages, including reusability, which enables more sustainable and environmentally friendly processes. Additionally, they contribute to minimizing waste generation and lowering energy consumption during the reaction [14].
The development of heterogeneous catalysts for the Guerbet reaction is relatively simple when considering hydrogenation and dehydrogenation processes, as metal-based catalysts can be tailored to efficiently drive these transformations [15]. However, incorporating both acidic and basic surface properties into solid materials, essential for aldol condensation catalysis and closely linked to the presence of metal oxides, remains a more complex challenge. In essence, the design of multifunctional catalysts must integrate both metallic sites and acidic/basic metal oxides, ensuring that basic sites selectively promote acetaldehyde condensation, acidic sites facilitate the dehydration of 3-hydroxybutanal, and metallic species enable ethanol dehydrogenation as well as hydrogenation of 2-butenal and butyraldehyde (see Scheme 1). It is important to note that these active sites, located on the same solid surface, may operate synergistically during the reaction or function independently at different stages [2,3].
It is worth highlighting that heterogeneous catalysts featuring basic or acid/base sites, as reported in the literature, typically require operating temperatures above 573 K [1,2,9,10,16]. These include basic metal oxides, modified zeolites, zirconia-based materials, alkaline compounds on inert carbon supports, and hydrotalcites, among others. Interestingly, when studying the catalytic coupling of ethanol into butanol over MgO-based catalysts supported on activated carbons, it has been highlighted that this type of supports can enhance water tolerance and improve process selectivity towards butanol production [17], enabling the operation with ethanol/water mixtures. However, the introduction of metallic species into such catalytic systems can substantially lower the reaction temperature, reaching values as low as 443 K, while maintaining similar activity and selectivity towards 1-butanol [16]. A study [18] attributes this temperature reduction, compared to mixed oxide catalysts, to the role of metals in ethanol dehydrogenation, which is recognized as the rate-determining step in the Guerbet reaction. Several investigations highlight the advantages of employing copper and nickel as active metal phases in multicomponent catalysts, given their hydrogenation and dehydrogenation capabilities [4,15,19,20,21,22]. As an example of catalytic performance, a high surface area Cu/CeO2 catalyst [4] achieved a 1-butanol selectivity of 60% with 16% ethanol conversion at 463 K.
The aim of this academic study is to combine the advantages of metallic phases (Cu and Ni) with the well-known basic character of MgO. All components will be dispersed on high-surface-area graphite (HSAG) to maximize the exposure of active phases. Given its weak interaction with these components, HSAG facilitates closer contact between the active phases, enhancing their interaction and catalytic performance [23]. In addition, the inert and non-microporous HSAG support would improve the catalytic activity due to its hydrophobic nature, as water is a by-product of the Guerbet reaction. Finally, an exhaustive characterization has been carried out on all the designed catalysts, in which compositional variations were introduced to evaluate their influence on reaction pathways. This analysis aims to correlate the physicochemical surface properties of the materials with their observed performance. In general, the objective is to determine the quantity and nature of acidic and basic surface sites, as well as the number and distribution of metallic nanoparticles.

2. Results

2.1. Characterization of the Multicomponent Catalysts

The BET surface area (SBET) of the bare HSAG support and several reduced catalysts is presented in Table 1. The introduction of magnesium oxide (MgO) leads to a reduction in surface area, dropping from 396 m2/g for HSAG to 317 m2/g for Mg-HSAG. Notably, the total magnesium content incorporated onto the support is approximately 1 wt.%. The inclusion of metallic components further decreases the SBET values, as observed in 5Cu-Mg/HSAG (270 m2/g) and 4Cu1Ni-Mg/HSAG (243 m2/g). This decline is likely due to the presence of MgO and metal nanoparticles, whose crystallites may block the holes between aggregated graphite grains. Moreover, the mobility of the added elements during reduction treatment is expected to influence the surface area of the catalysts. For instance, the 4Cu1Ni-Mg/HSAG sample subjected to pre-treatment in helium at 723 K before hydrogen reduction exhibited an even lower BET area (214 m2/g) compared to the 4Cu1Ni-Mg/HSAG catalyst reduced directly in H2 at 573 K. These findings align with previously reported trends for metallic systems supported on graphite [24]. Additionally, the observed changes suggest that both the promoter phase (MgO) and the metallic species are well dispersed, indicating the formation of small crystallites within these materials. The precise particle sizes will be further examined using physicochemical techniques, including X-ray diffraction (XRD) and transmission electron microscopy (TEM).
Figure 1 presents the H2 temperature programmed reduction (H2-TPR) profiles of all synthesized catalysts. The 5Cu/HSAG sample displays a reduction peak around 490 K, whereas 5Ni/HSAG exhibits one at 550 K, followed by a broader peak spanning 580–690 K. These findings align with previously reported data [25]. Additionally, the broad peak observed at higher temperatures in the H2-TPR profiles of the Ni-based catalysts supported on carbon can be attributed to the reduction of larger particles of NiO particles and the gasification of surrounding carbon atoms facilitated by the Ni nanoparticles [26]. Notably, the bimetallic 4Cu1Ni/HSAG sample exhibits an intermediate behavior, characterized by a single, sharp reduction peak at 515 K, with no visible broad peak typically associated with nickel. The absence of this broad feature and the downward shift in temperature, compared to the sharp peak of 5Ni/HSAG at 550 K, indicates the formation of bimetallic Cu-Ni nanoparticles with modified reduction characteristics. This observation is consistent with previous studies that describe the behavior of Cu and Ni nanoparticles and their enhanced reducibility [20,27].
On the other hand, the H2 TPR profiles of the monometallic catalysts containing magnesium indicate that the reduction of metal precursors takes place at higher temperatures compared to their magnesium-free counterparts. This temperature shift is particularly evident in the 5Cu-Mg/HSAG sample, where the reduction peak appears at 530 K, in contrast to 490 K for 5Cu/HSAG. Similarly, the 5Ni-Mg/HSAG catalyst exhibits a main reduction peak at 560 K, whereas its non-promoted version, 5Ni/HSAG, shows this peak at 550 K. Regarding the broad peak observed in the 600–700 K range for Ni-containing catalysts, this feature is also present in the H2 TPR profiles of the bimetallic samples. Notably, the sharp reduction peak progressively shifts to higher temperatures as the nickel content increases: 530 K for 4.75Cu0.25Ni-Mg/HSAG, 540 K for 4Cu1Ni-Mg/HSAG, and 555 K for 2.5Cu2.5Ni-Mg/HSAG, but always remains lower than that of the 5Ni-Mg/HSAG sample. These shifts in the reduction peaks for the bimetallic catalysts confirm that copper facilitates the reduction of nickel nanoparticles. Our results are consistent with previously reported TPR studies in the literature [27]. In particular, research on multi-walled carbon-nanotubes-supported catalysts containing nickel, copper, and magnesia nanoparticles has shown that the presence of MgO enhances the interaction of both metals with the support. Based on these H2 TPR findings, the reduction treatment prior to catalytic testing was set at 573 K for the Cu-based catalysts and 723 K for Ni-containing catalysts.
Figure S1 (available in the Supplementary Information) presents the X-ray diffractograms of the HSAG support and the reduced catalysts. No diffraction peaks associated with magnesium oxide were detected, which is expected given that all samples contain approximately 1.45 wt.% of Mg. All catalysts exhibited diffraction peaks characteristic of HSAG at 2θ = 26.2°, 43.9°, 54.6°, and 77.6°. However, several XRD signals corresponding to copper and nickel species appear near the broad carbon (HSAG) peak at 43.9°, making it difficult to distinguish the primary diffraction peaks of metallic Cu and Ni, located at 43.5° and 44.4°, respectively. Nevertheless, the most intense reflection of metallic Cu was faintly discernible in the 5Cu/HSAG and 4Cu1Ni/HSAG samples (Figure S1b,d). In contrast, the reflections at 2θ = 43.5°, 50.4°, and 74° were identified in all Cu-containing catalysts supported on Mg/HSAG (Figure S1e–h). Regarding Ni, a diffraction peak corresponding to metallic nickel was only detected in the 5Ni/HSAG catalyst (Figure S1c) at 2θ = 44.4°. These variations may be attributed to the MgO promoter and its effect on the metal nanoparticle size. While Ni appears well dispersed regardless of its loading, Cu nanoparticles seem to increase in size when MgO is present. Additionally, very small peaks associated with copper oxide (35.7° and 38.9°) and nickel oxide (37.4° and 43.5°) were only observed in the XRD patterns of the 5Cu/HSAG, 5Cu-Mg/HSAG, 4Cu1Ni-Mg/HSAG, and 5Ni-Mg/HSAG catalysts. This suggests that the smaller metallic particles in these catalysts may have undergone oxidation after exposure to ambient air.
Figure S2 (available in the Supplementary Information) presents the XRD profiles of the 4Cu1Ni-Mg/HSAG catalyst following reduction in hydrogen at 573 K (Figure S2b) and after pre-treatment in helium at 723 K (Figure S2d). Additionally, the XRD patterns of these two catalysts after reaction are shown in Figure S2c and Figure S2e, respectively. Notably, in the 4Cu1Ni-Mg/HSAG sample reduced in H2, the reflections associated with CuO (35.7° and 38.9°) disappear after reaction. Furthermore, in the catalyst subjected to helium treatment (Figure S2d), as well as after reaction (Figure S2e), the diffraction peaks corresponding to metallic Cu (43.5°, 50.4°, and 74°) become sharper. This suggests that metallic copper is significantly influenced by both the high-temperature inert gas treatment and the reaction conditions. The XRD patterns of the remaining spent catalysts are provided in Figure S3. The hypothesis that Cu tends to form metallic aggregates after reaction at 503 K is supported by the fact that the Cu0 peaks are more intense in the spent catalysts compared to their counterparts reduced only in H2. Additionally, the diffraction peaks corresponding to metallic Ni (Ni0) and nickel oxide (NiO) are undetectable in all Ni-containing catalysts, indicating that Ni0 particles do not undergo the same sintering effects as those observed for Cu0.
The findings from the transmission electron microscopy (TEM) analysis are summarized in Figure 2 and Figure 3, which present selected images along with histograms illustrating the particle size distribution of the catalysts. It is important to highlight that TEM alone cannot distinguish between Cu and Ni particles; therefore, compositional data were obtained using an EDX detector integrated into the TEM/STEM system. Nevertheless, the TEM micrographs confirm the presence of well-dispersed metal nanoparticles (NPs) with diameters ranging from 4 to 10 nm following hydrogen reduction. This observation suggests the formation of active Cu0 and/or Ni0 species. The histograms, generated after analyzing a statistically significant number of nanoparticles, reveal that all reduced monometallic catalysts exhibit a nearly Gaussian particle size distribution. In contrast, the bimetallic catalysts and the spent samples display a long-tail distribution, indicating the presence of larger particles.
Several key insights can be obtained from the analysis of Figure 2 and Figure 3. First, the presence of magnesium appears to have a negative impact on copper particle size, as all Cu-containing catalysts exhibit larger particles compared to their magnesium-free counterparts, 5Cu/HSAG and 4Cu1Ni/HSAG, which show sizes of 4.7 nm and 4.2 nm, respectively (Figure 2a,c). A similar trend is observed in Ni-based catalysts, where a comparison between 5Ni/HSAG and 5Ni-Mg/HSAG reveals particle sizes of 4.9 nm and 4.5 nm, respectively (Figure 2b and Figure 3b). In contrast, bimetallic Cu-Ni catalysts exhibit smaller particle sizes as the Ni content increases: 6.8 nm for 4.75Cu0.25Ni-Mg/HSAG, 6.6 nm for 4Cu1Ni-Mg/HSAG, 5.8 nm for 2.5Cu2.5Ni-Mg/HSAG. Additionally, the average particle size increases for all catalysts after reaction (Figure 2 and Figure 3, right) when compared to their reduced counterparts. Interestingly, the introduction of nickel effectively mitigates copper sintering to a certain extent, with 1 wt.% Ni being the optimal loading to minimize Cu agglomeration. For the 4Cu1Ni-Mg/HSAG* sample, which underwent thermal treatment under an inert atmosphere at 723 K before reduction, TEM analysis indicated further particle sintering (8.8 nm, Figure 3f left), with approximately 20% of the particles exceeding 15 nm. Notably, after reaction, the fraction of larger particles detected in 4Cu1Ni-Mg/HSAG* decreased to below 10%, suggesting that some nanoparticle redispersion might occur during the catalytic process.
Energy-dispersive X-ray spectroscopy (EDX) elemental mapping was conducted to examine the potential interactions between Cu, Ni, and Mg, as well as to assess the distribution of nanoparticles across the support material. The EDX images of the monometallic catalysts (see Figure S7 in the Supplementary Information) revealed that the active phases (magnesium, copper, and nickel) were highly dispersed over the carbon support, sharing the same spatial distribution. The bimetallic Cu-Ni catalysts, both in their reduced state (Figure S8) and after reaction (Figure 4), were also analyzed via EDX to further investigate the interaction between Cu and Ni and evaluate the extent of copper sintering. The samples reduced at 573 K displayed copper particles with a heterogeneous size distribution, whereas nickel and magnesium remained well dispersed. Interestingly, despite the variations in copper particle size, their spatial distribution coincided with that of Ni and Mg, suggesting the formation of Cu-Ni particles dispersed over Mg/HSAG. After reaction (Figure 4), the Cu-Ni particles appeared larger, with a notable increase in particle size. Even in the catalyst with equal Cu and Ni content (2.5Cu2.5Ni-Mg/HSAG, Figure 4a), the smaller particles exhibited a higher Ni concentration. These observations suggest that sintering during reaction primarily affects Cu nanoparticles. This surface reorganization and aggregation of Ni and Cu nanoparticles appears to take place not only during the early stages of reduction but also throughout the reaction process. As will be discussed later, before reaching steady-state conditions and achieving stable conversion levels, all catalysts require 3–4 h under reaction conditions.

2.2. Characterization of the Catalytic Surfaces

The surface properties of the most relevant catalysts were analyzed using temperature-programmed desorption of ammonia (NH3 TPD) to determine the distribution of acidic sites. The corresponding NH3 TPD profiles are displayed in Figure S4. As illustrated in Figure S5, which presents an example of NH3 TPD profile fitting, these catalysts exhibit acidic sites of varying strengths, as each profile can be deconvoluted into three distinct components, with maxima at approximately 450 K, 525 K, and 625 K. The desorption temperature ranges have been associated with weak acid sites (350–500 K), medium-strength sites (500–550 K), and strong acid sites (550–675 K). This correlation between desorption temperature and acid strength aligns with previously reported trends for Cu/ZnO catalysts, as well as for materials supported on SiO2 and Al2O3 [13,28,29]. The final acidity distribution data are summarized in Table 2, which also includes, in column 5, the integrated areas under each TPD profile (normalized area, arbitrary units) to facilitate comparison of total acidity levels.
An increase in the number of surface acid sites can be observed in Table 3 when comparing the bifunctional catalysts (containing MgO) with the monometallic 5Cu/HSAG and 5Ni/HSAG samples. Previous studies have reported ammonia adsorption on metal surfaces such as Cu and Ni [28], with a general agreement regarding the nature of metal–ammonia interactions, where NH3 molecules bind to the surface through their nitrogen atom. Furthermore, it has been established that metallic Cu nanoparticles can behave as Lewis acids due to their electron-deficient character [30]. The 5Cu-Mg/HSAG and 5Ni-Mg/HSAG samples exhibit NH3 TPD profiles similar to those of their MgO-free counterparts (5Cu/HSAG and 5Ni/HSAG), despite their higher total acidity values when supported on Mg/HSAG. This suggests that the bifunctional catalysts integrate both the metallic sites and the acid centers of MgO. Among them, 5Cu-Mg/HSAG displays the highest total acidity (Table 3). Regarding the distribution of acid site strengths, weak acid sites predominate in the Mg/HSAG and 4Cu1Ni-Mg/HSAG samples (38% and 40%, respectively), while strong acid sites are more prevalent in 5Ni-Mg/HSAG and 4Cu1Ni-Mg/HSAG* (39% and 41%, respectively). Additionally, as seen in Table 3, the presence of nickel in bimetallic catalysts (4Cu1Ni-Mg/HSAG and 4Cu1Ni-Mg/HSAG*) leads to a reduction in total acidity compared to the monometallic bifunctional catalysts, particularly in 5Cu-Mg/HSAG.
The strength distribution and quantity of basic sites exposed on the catalyst surfaces were evaluated through adsorption microcalorimetry using carbon dioxide as a molecular probe. Figure S6 presents the differential heat of CO2 adsorption as a function of surface coverage, while Table 4 summarizes the key findings. The classification of basic site strength follows these criteria: strong (Qdiff > 110 kJ/mol), medium (110 > Qdiff > 60 kJ/mol), and weak (Qdiff < 60 kJ/mol), with the threshold between chemical and physical CO2 adsorption set at 40 kJ/mol [31]. Notably, the Mg/HSAG sample exhibits a similar strength distribution and adsorption heat values before and after thermal treatment at 723 K (Mg/HSAG*), although the latter retains a higher total amount of adsorbed CO2 (Table 4). The lower total number of basic sites observed in Mg/HSAG may be attributed to CO2 and water adsorbed from ambient exposure, which are eliminated upon in situ thermal treatment at 723 K, leading to enhanced basicity. Interestingly, in 5Cu-Mg/HSAG, both the total number and percentage of strong basic sites decrease compared to Mg/HSAG. However, for 5Ni-Mg/HSAG, the percentage of strong sites is significantly higher, with a total basic site count comparable to that of Mg/HSAG after thermal treatment, as the Ni-based sample was reduced at 723 K. To further explore the basicity differences between Cu and Ni when supported on Mg/HSAG, the bimetallic 4Cu1Ni-Mg/HSAG catalyst was also analyzed. Interestingly, this sample shows a sharp decline in the percentage of strong basic sites (8%, Table 4) compared to the monometallic catalysts (12% for Cu and 38% for Ni). However, its total basic site content (70 µmol CO2/g) is not significantly lower than that of 5Cu-Mg/HSAG (54 µmol CO2/g). Notably, when 4Cu1Ni-Mg/HSAG undergoes helium treatment at 723 K, the initial adsorption heat values increase at low CO2 surface coverage, which correspond to 18% of strong basic sites, while the total number of basic sites remains unchanged. This increase in basic strength, also observed in 5Ni-Mg/HSAG, has been linked to metal–MgO interactions at elevated temperatures, as the catalysts reduced or treated at 723 K consistently exhibit higher basic site strength. At this stage, it is important to acknowledge the inherent limitations of this method for assessing surface basicity. Since the catalysts are either reduced or thermally treated in a reactor and subsequently exposed to air before the adsorption experiments, there is a risk of oxidation, hydration, or carbonation. Even though the samples undergo high-temperature outgassing before microcalorimetric chemisorption, their surface composition under reaction conditions differs from that during the Guerbet process. Nonetheless, these experiments indicate that interactions among catalyst components can significantly impact basicity measurements.

2.3. Comparative Study of Catalytic Materials in the Ethanol to Butanol Reaction

The effect of reaction temperature on 1-butanol selectivity in a continuous-flow reactor was evaluated in the 443–573 K range for the 5Cu-Mg/HSAG catalyst, as illustrated in Figure S9. While ethanol conversion rises with increasing temperature, reaching 70% at 573 K, the highest 1-butanol selectivity (26%) is observed at 503 K. To further investigate the impact of temperature on product selectivity, Figure S10 presents the distribution of reaction products (1-butanol, acetaldehyde, 1,1-diethoxyethane, 2-butanone, 1-hexanol, and diethyl ether) for the 5Cu-Mg/HSAG catalyst. The selectivity trends observed in this study closely resemble those previously reported in the literature under similar temperature conditions using Cu- and Ni-based catalysts [8,9,32]. These selectivity patterns can be explained by the formation of a primary product in a stepwise reaction, which subsequently leads to undesired by-products at higher temperatures. The decline in acetaldehyde concentration above 503 K, shown in Figure 2, can also be attributed to its further transformation in the reaction pathway. Additionally, selectivity to 1,1-diethoxyethane increases to 23%, consuming acetaldehyde, as this compound is produced via acid-catalyzed acetylation of acetaldehyde and ethanol [20]. Other minor by-products with selectivity below 10% include 2-butanone, 1-hexanol, and diethyl ether. Finally, the selectivity of unidentified products, which remains below 5% at temperatures under 503 K, shows a sharp increase to 24% at 573 K. In conclusion, the product distribution at temperatures below 503 K supports the proposed reaction mechanism depicted in Scheme 1.
Since the highest 1-butanol selectivity was observed at 503 K (Figure S9), with high ethanol conversion and a carbon balance exceeding 90% in all cases, the catalyst screening was conducted at this temperature. Prior to these tests, blank reactions were carried out using HSAG as a support and Mg/HSAG, both of which exhibited negligible ethanol conversion at 503 K. Table 4 provides a summary of the catalytic performance of the various samples after 24 h, once the steady-state conditions were established.
As shown in Table 4, the composition of the catalysts plays a crucial role in determining the final product distribution. The identified products include 1-butanol, acetaldehyde, 1,1-diethoxyethane, carbon monoxide, methane, and several minor by-products, such as 2-butenal, 2-butanone, ethyl acetate, ethylene, 1-hexanol, butanal, diethoxybutane, 1-octanol, 2-butanol, 2-ethyl-1-butanol, 2-ethyl-1-hexanol, and acetone. While CH4 and CO are formed through acetaldehyde decarbonylation, a reaction facilitated by metallic catalysts, acetone is generated via ketonization of acetic acid, which itself results from the hydrolysis of ethyl acetate [3,33]. Notably, after 24 h of reaction, the catalysts demonstrated high stability in terms of 1-butanol selectivity, as depicted in Figure S11. This figure also highlights that conversion and butanol selectivity undergo changes during the initial 5–6 h of reaction. After this period, a slight deactivation is observed, though the butanol selectivity values remain stable. The initial fluctuations in conversion and selectivity suggest that the catalyst surfaces undergo modifications during the early stages of reaction at 503 K and 50 bar. Moreover, a comparison of average particle sizes between fresh and spent catalysts, summarized in Table 4, confirms that some degree of nanoparticle sintering occurs throughout the reaction.

3. Discussion

To better understand the synergistic effect between the metallic components (Cu and/or Ni) and magnesium oxide, it is essential to first evaluate their individual catalytic behavior. Therefore, the catalysts 5Cu/HSAG, 5Ni/HSAG, and 4Cu1Ni/HSAG were tested in the absence of MgO, showing high ethanol conversion but low 1-butanol selectivity (4%, 7%, and 5%, respectively, Table 4). Notably, the main by-products differed between the monometallic catalysts. Thus, for 5Cu/HSAG, the dominant by-products were acetaldehyde and 1,1-diethoxyethane. In contrast, 5Ni/HSAG predominantly formed methane, carbon monoxide, diethyl ether, and acetone. As expected, the 4Cu1Ni/HSAG catalyst exhibited an intermediate behavior, with acetaldehyde, 1,1-diethoxyethane, methane, carbon monoxide, and acetone as the primary side products. The formation of acetaldehyde via ethanol dehydrogenation represents the initial step of the Guerbet reaction. Interestingly, its selectivity varies among catalysts: 52% for 5Cu/HSAG, 24% for 5Ni/HSAG, and 39% for 4Cu1Ni/HSAG. This trend aligns with the literature, as copper is well known for its ability to catalyze ethanol dehydrogenation to acetaldehyde [9]. Conversely, the catalyst Mg/HSAG showed negligible ethanol conversion under these experimental conditions. This confirms that the role of MgO is to provide acid/base sites, as identified in the characterization analyses (Table 2 and Table 3), which facilitate the condensation of acetaldehyde into 1-butanol.
As anticipated, the bifunctional catalysts significantly enhanced 1-butanol selectivity compared to their magnesium-free counterparts. The selectivity towards 1-butanol increased from 4% to 26% when comparing Cu/HSAG with 5Cu-Mg/HSAG, and from 7% to 24% for Ni/HSAG versus 5Ni-Mg/HSAG (Table 4). This improvement is even more pronounced in the bimetallic Cu-Ni systems, where 1-butanol selectivity reaches 31% for 4.75Cu0.25Ni-Mg/HSAG, 32% for 2.5Cu2.5Ni-Mg/HSAG, and 39% for 4Cu1Ni-Mg/HSAG, in contrast to 4Cu1Ni/HSAG, which only attained 5%, despite exhibiting similar ethanol conversion levels. The enhanced 1-butanol selectivity observed for the M-Mg/HSAG catalysts could be attributed to the synergistic interaction between exposed metal sites, which are crucial for dehydrogenation/hydrogenation reactions, and an optimal acid/base site balance. The latter plays a key role in aldol condensation (R2) and the dehydration of 3-hydroxybutanal (R3) (Scheme 1). Although it is well established in the literature that high-strength basic sites contribute to 1-butanol selectivity [2,5,9], our findings indicate that an optimal combination of medium–high strength basic sites along with a moderate concentration of acid sites is preferable to achieving higher 1-butanol yields. To further support this hypothesis, it is worth noting that 5Ni-Mg/HSAG, despite having the highest number of basic sites (86 µmol CO2 chemisorbed/g, 38% high-strength sites, Table 3) and the highest total acidity (39% strong acid sites, Table 2), exhibits the lowest 1-butanol selectivity (23%). Similarly, the 5Cu-Mg/HSAG catalyst, which contains 35% strong acid sites (Table 3) and the smallest amount of basic sites (54 µmol CO2 chemisorbed/g, 12% high-strength sites, Table 2), reaches only 26% selectivity towards 1-butanol (Table 4). In contrast, 1-butanol selectivity improves significantly in the 4Cu1Ni-Mg/HSAG sample (39%, Table 4) compared to the monometallic catalysts. This catalyst exhibits intermediate basicity (70 µmol CO2 chemisorbed/g, 8% high strength sites, 77% medium strength sites, Table 2) between 5Ni-Mg/HSAG and 5Cu-Mg/HSAG, along with a moderate acid site concentration (40% weak acid sites, Table 3).
Notably, the Cu-Ni ratio, along with the presence of magnesium in the catalyst formulation, significantly influences the acid/base site balance and, consequently, 1-butanol selectivity. For example, Cu/HSAG and 5Cu-Mg/HSAG produce higher amounts of 1,1-diethoxyethane (31% and 15%, respectively) compared to catalysts containing only nickel (Ni/HSAG and 5Ni-Mg/HSAG), where the proportion of this compound remains below 2% in the product stream. It is worth mentioning that magnesium incorporation reduces the formation of 1,1-diethoxyethane compared to Cu/HSAG, likely due to the introduction of basic sites and the partial coverage of HSAG acid sites. Since these acid sites are necessary to catalyze the acetylation of acetaldehyde with ethanol, leading to 1,1-diethoxyethane, their reduction explains the observed selectivity shift. Regarding the bimetallic Cu-Ni catalysts, increasing Ni content in combination with Mg further suppresses 1,1-diethoxyethane formation. Its selectivity decreases from 13% for 4Cu1Ni/HSAG to 10% for 4.75Cu0.25Ni-Mg/HSAG, then to 4% for 4Cu1Ni-Mg/HSAG, and finally to 2% for 2.5Cu2.5Ni-Mg/HSAG (Table 4). However, higher nickel concentrations tend to promote the formation of undesirable by-products such as carbon monoxide, methane, and acetone. Specifically, CH₄ and CO selectivities decrease from 25% and 15% for both Ni/HSAG and 5Ni-Mg/HSAG, to 15% and 11% for 2.5Cu2.5Ni-Mg/HSAG, then to 6% and 4% for 4Cu1Ni-Mg/HSAG, and finally to 2% and 2% for 4.75Cu0.25Ni-Mg/HSAG. The synergistic interaction between the metals and magnesium is further demonstrated by comparing the CH₄ and CO selectivities of 4Cu1Ni/HSAG (12% and 8%) versus 4Cu1Ni-Mg/HSAG (6% and 4%). Additionally, in nickel-containing catalysts, the presence of Mg helps suppress the formation of acetone, another unwanted side product. This effect is particularly evident as acetone selectivity drops from 9% with Ni/HSAG and 7% with 4Cu1Ni/HSAG to trace amounts in the bifunctional catalysts. Finally, based on the influence of metal composition on product distribution and 1-butanol selectivity, the optimal Cu-Ni ratio appears to be 4 wt.% Cu and 1 wt.% Ni.
By comparing the transmission electron microscopy (TEM) results (Figure 2 and Figure 3) with the catalytic performance of each sample, it appears that metal particle size does not play a critical role in determining ethanol conversion levels. However, the reduced 4Cu1Ni-Mg/HSAG* catalyst (treated in helium at 723 K) exhibits the largest particle size (9.7 nm after reaction), which seems to have a slightly negative impact on conversion. The ethanol conversion decreases from 24% with 4Cu1Ni-Mg/HSAG (7.1 nm after reaction) to 20% for 4Cu1Ni-Mg/HSAG*. In contrast, 1-butanol selectivity is significantly enhanced for the 4Cu1Ni-Mg/HSAG* catalyst. This observation aligns with the previously mentioned synergistic effect of the optimal Cu-Ni ratio (4 wt.% Cu and 1 wt.% Ni), which, when combined with medium–high strength basic sites and moderate acidity, leads to a notable increase in 1-butanol yield. To further support this conclusion, it is important to highlight that 4Cu1Ni-Mg/HSAG* contains predominantly medium–high strength basic sites (18% strong, 70% medium, Table 3) and has lower total acidity (41% strong acid sites, Table 2) compared to 4Cu1Ni-Mg/HSAG, which primarily features weak acid sites (40%, Table 2).
These findings reinforce the synergistic effect between metallic components, as the optimal Cu and Ni loading, and the acid/base sites introduced by magnesium oxide, results in a catalyst with a well-balanced surface acidity and basicity. This balance plays a crucial role in enhancing 1-butanol selectivity. Taking into consideration the previously discussed points and the fact that Mg/HSAG alone is unable to convert ethanol into 1-butanol, it is evident that the metal function of the catalyst plays a significant role in 1-butanol yields. Consequently, the site time yield towards 1-butanol was calculated (Figure S12) for the bifunctional catalysts, based on the number of exposed metal atoms per gram of catalyst. Thus, Figure S12 illustrates that the 4Cu1Ni-Mg/HSAG* catalyst, after thermal treatment in helium at 723 K, significantly promotes the reaction pathway towards 1-butanol due to the optimal combination of exposed copper and nickel active sites. Interestingly, and contrary to our catalytic results, Patel et al. [34] found that ethanol conversion and 1-butanol selectivity were not influenced by the addition of Cu (via impregnation with copper nitrate) to the MgO/CNF system. They reported a 1-butanol selectivity of 30% ethanol using a Cu/MgO catalyst supported on CNF in a plug flow reactor at 573 K and atmospheric pressure. However, they observed a sharp decline in activity (conversion to 1%) within the first hour of reaction, in contrast to our HSAG-supported catalysts, which maintained high stability for 24 h. The cooperative interaction among metallic, acidic, and basic sites in the Guerbet reaction has been previously described by Benito et al. [21], who reported 32.4 mol% ethanol conversion and 52.1 mol% 1-butanol selectivity using Cu/Mg/Al hydrotalcite-derived mixed oxide catalysts in a batch reactor at 503 K and 40 bar after 12 h. Notably, our continuous flow reactor achieves a comparable 1-butanol yield under the same temperature and pressure conditions, while also eliminating the need for catalyst regeneration treatments before reuse. From a practical application perspective, due to the sequential nature of the reaction mechanism (Scheme 1), maintaining high selectivity to 1-butanol at elevated ethanol conversion levels is inherently challenging. Therefore, while catalyst design plays a critical role, an integrated reactor design is equally necessary. This includes strategies to facilitate product separation and purification, along with reactant recirculation [35]. A key by-product that significantly impacts reaction efficiency is water. The presence of water not only affects catalytic performance but also leads to structural modifications in the active phases, as observed in the nanoparticle size changes determined by TEM (Table 4) when comparing freshly reduced catalysts and those subjected to 24 h of reaction. These transformations are particularly evident during the initial reaction stages, as seen in Figure S11. To counteract the detrimental role of water, this study explored hydrophobic support materials, such as graphite, to stabilize the molecular components of the catalyst. However, the expected improvements in butanol yield remained limited. Given the importance of by-product removal during reaction, alternative hydrophobic supports should be further investigated in future studies. The ultimate goal should be to achieve a molecular level design of catalytic materials, integrating nanoscopic active components with macroscopic structured or surface-modified supports, to enhance catalytic efficiency in this process.

4. Materials and Methods

Since the majority of the materials and methods have been extensively detailed in the Lopez-Olmos PhD dissertation [36], which is publicly accessible, this section will provide a concise overview of the most relevant aspects directly related to the present study. The catalysts were synthesized using the incipient wetness impregnation method, employing aqueous solutions of nitrate precursors for active components and HSAG (SBET = 396 m2/g, provided by Timcal, Bodio, Switzerland) as the support material. The component weights were adjusted to achieve an atomic metal/Mg ratio of 2:1. The resulting Mg impregnated HSAG material was subjected to drying at 383 K, followed by calcination at 873 K under a helium atmosphere, leading to the formation of magnesium oxide. All the metal precursor-impregnated materials were dried at 383 K and, prior to reaction, were activated in situ under a hydrogen flow (20 mL STP/min) for 1 h at 573 K, except for Ni/HSAG and Ni-Mg/HSAG, which were reduced at 723 K.
TPR profiles were recorded using a home U-shaped quartz reactor equipped with a thermal conductivity detector (TCD). The experiments were performed with a gas flow of 30 mL/min, consisting of 4 vol.% H2 in He, and a heating rate of 10 K/min, starting from room temperature up to 973 K.
X-ray diffraction (XRD) patterns were recorded over a 2θ range of 4° to 90°, with a step size of 0.04°/s, using a Polycristal X’Pert Pro PANalytical (Almelo, The Netherlands) diffractometer. The system operated with Ni-filtered Cu Kα radiation (λ = 1.54 Å) under conditions of 45 kV and 40 mA.
The measurement of the specific surface area (SBET) was carried out using nitrogen adsorption–desorption isotherms recorded at 77 K with a Micromeritics (Norcross, GA, USA) ASAP 2010 automated volumetric adsorption system. Prior to analysis, the samples underwent in situ degassing at 423 K for 5 h.
The acidity of the catalysts was determined through NH3 TPD (temperature-programmed desorption of ammonia) using a Micromeritics Autochem II 2920 analyzer with a thermal conductivity detector (TCD). Prior to the test, the samples underwent in situ reduction at the respective reduction temperature in a hydrogen atmosphere, followed by helium purging and exposure to 5% ammonia gas at 323 K. Subsequently, another helium purge was performed at 373 K before applying a heating rate of 10 K/min from 373 to 623 K.
Carbon dioxide was employed as a probe molecule to evaluate the basic character of the solid surface. Before CO2 adsorption, the samples underwent in situ reduction at their respective reduction temperature (573 K or 723 K) and were subsequently degassed under vacuum at 623 K for 10 h. The amount of CO2 adsorbed was determined using a volumetric analysis system, where the adsorption cell was placed inside a differential heat-flow microcalorimeter (Tian-Calvet C80, Setaram, Ecully, France) maintained at 323 K. CO2 pulses were introduced sequentially, allowing for the equilibration of both pressure and heat-flow in the calorimeter. By integrating the heat-flow signal for each injection and knowing the amount of CO2 adsorbed, the differential heats of adsorption (Qdiff) were estimated.
TEM images and elemental mappings were acquired using a JEOL (Tokyo, Japan) JEM-2100F transmission electron microscope operated at 200 kV and equipped with an energy-dispersive X-ray (EDX) detector. Before analysis, the samples were dispersed in ethanol through ultrasonic treatment and deposited on a gold grid (Aname, Lacey carbon, 200 mesh). For the scanning transmission electron microscopy (STEM, JEOL, Tokyo, Japan) measurements, a 1 nm spot size was selected.
Ethanol condensation reaction was evaluated in a continuous stainless-steel fixed-bed reactor, using the same conditions: gas phase at 50 bar and 503 K, for 24 h. The fixed-bed reactor is a tube of stainless steel 316 L (52.5 cm long, 0.049″ wall diameter and 3/8″ outer diameter). In a preliminary study, a set of experiments were performed to assess the impact of temperature using 5Cu-Mg/HSAG as catalyst in the range 443-573 K, concluding that 503 K is the optimal reaction temperature in term of conversion and 1-butanol selectivity to butanol. That is, at lower reaction temperatures low conversions (<10%) are obtained with problems of reproducibility, while at higher temperatures the selectivity to butanol decreases due to secondary reactions. Prior to the reaction, an aliquot of each sample (0.5 g) was reduced under flowing hydrogen at 573 K (1 h) in the case of all Cu containing samples, while the samples with monometallic Ni were reduced at 723 K (1h). With the aim of understanding the effect of the reduction pre-treatment, the sample 4Cu1Ni-Mg/HSAG was also tested after a treatment at 723 K (1 h) under flowing helium before the hydrogen reduction at 573 K. In all these catalytic experiments, the reactant feed consisted of a helium flow (50 mLSTP/min) and vaporized ethanol (0.02 mL/min), which was supplied by a precise HPLC pump. Weight hourly space velocity (WHSV) in ethanol was 1.9 h−1.
The gaseous products were analyzed with two on-line gas chromatographs (GC) fitted with FID and TCD detectors, while the condensed reaction products were collected and analyzed with another GC equipped with an FID detector. Commercial standards were used for calibration of the reaction products. All the information about the equations and method of calculating the ethanol conversion, the selectivity to a specific product, the carbon balance, and the site time yield (STY) towards 1-butanol, can be found in the Supplementary Information File. The comparative of STY values of bimetallic catalysts is represented in Figure S12 as well in the graphical abstract.

5. Conclusions

Bifunctional heterogeneous catalysts composed of Cu-Ni nanoparticles supported on HSAG, along with magnesium oxide, facilitate ethanol conversion via the Guerbet pathway. The synergy between metal sites, which promote dehydrogenation/hydrogenation reactions, and well-balanced acid/base sites, responsible for aldol condensation and dehydration, enhances selectivity towards 1-butanol. Catalyst characterization reveals that surface properties influence catalytic performance. The presence of MgO, in combination with Cu-Ni nanoparticles, improves both ethanol conversion and selectivity. Among the tested catalysts, 4Cu1Ni-Mg/HSAG* treated in helium at 723 K exhibited the highest 1-butanol yield under stable conditions (20% ethanol conversion, 44% selectivity). Therefore, the optimal catalytic performance was achieved with bimetallic Cu-Ni catalysts, where a well-adjusted Cu/Ni ratio, combined with well-balanced acid/base properties, favored higher 1-butanol yields.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15030272/s1, Figures S1–S3: XRD patterns of the support and catalysts (reduced and used); Figures S4 and S5: NH3-TPD profiles of the reduced catalysts; Figure S6: CO2 adsorption microcalorimetry at 323 K; Figures S7 and S8: EDX mappings of reduced catalysts; Figures S9 and S10: catalytic properties versus reaction temperature; Figure S11: Evolution catalytic properties with reaction time; Figure S12: Site time yield (STY, s−1) towards 1-butanol. A description of the calculations for nanoparticle average diameter, ethanol conversion, selectivity to a specific product, and carbon balance is provided.

Author Contributions

C.L.-O. carried out the experiments, either preparation of catalytic materials as reaction tests, and wrote a draft of this manuscript; I.R.-R. edited and proofread the manuscript, and participate with A.G.-R. in conceiving the original research proposal; A.G.-R. wrote the final manuscript and supervised the discussion of the results. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by MICIU/AEI/10.13039/501100011033 and by FEDER, UE, through grant PID2023-146481OB-I00.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

C.L.-O. thanks the MECD of Spain for a FPU predoctoral grant FPU16/05131.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Lu, B.; Ma, S.; Liang, S.; Wang, Z.; Liu, Y.; Mao, S.; Ban, H.; Wang, L.; Wang, Y. Efficient Conversion of Ethanol to 1-Butanol over Adjacent Acid–Base Dual Sites via Enhanced C–H Activation. ACS Catal. 2023, 13, 4866–4872. [Google Scholar] [CrossRef]
  2. Onyestyák, G. Carbon supported alkaline catalysts for Guerbet coupling of bioethanol. Period. Polytech. Chem. Eng. 2018, 62, 91–96. [Google Scholar] [CrossRef]
  3. Sun, J.; Wang, Y. Recent Advances in Catalytic Conversion of Ethanol to Chemicals. ACS Catal. 2014, 4, 1078–1090. [Google Scholar] [CrossRef]
  4. Earley, J.H.; Bourne, R.A.; Watson, M.J.; Poliakoff, M. Continuous catalytic upgrading of ethanol to n-butanol and C4 products over Cu/CeO2 catalysts in supercritical CO2. Green Chem. 2015, 17, 3018–3025. [Google Scholar] [CrossRef]
  5. Galadima, A.; Muraza, O. Catalytic Upgrading of Bioethanol to Fuel Grade Biobutanol: A Review. Ind Eng. Chem. Res. 2015, 54, 7181–7194. [Google Scholar] [CrossRef]
  6. Wang, W.C.; Tao, L. Bio-aviation fuel conversion technologies. Renew. Sust. Energy Rev. 2016, 53, 801–822. [Google Scholar] [CrossRef]
  7. León, M.; Díaz, E.; Ordóñez, S. Ethanol catalytic condensation over Mg-Al mixed oxides derived from hydrotalcites. Catal. Today 2011, 164, 436–442. [Google Scholar] [CrossRef]
  8. Marcu, I.C.; Tichit, D.; Fajula, F.; Tanchoux, N. Catalytic valorization of bioethanol over Cu-Mg-Al mixed oxide catalysts. Catal. Today 2009, 147, 231–238. [Google Scholar] [CrossRef]
  9. Sun, Z.; Vasconcelos, A.C.; Bottari, G.; Stuart, M.C.A.; Bonura, G.; Cannilla, C.; Frusteri, F.; Barta, K. Efficient Catalytic Conversion of Ethanol to 1-Butanol via the Guerbet Reaction over Copper- and Nickel-Doped Porous. ACS Sustain. Chem. Eng. 2017, 5, 1738–1746. [Google Scholar] [CrossRef]
  10. Ho, C.R.; Shylesh, S.; Bell, A.T. Mechanism and Kinetics of Ethanol Coupling to Butanol over Hydroxyapatite. ACS Catal. 2016, 6, 939–948. [Google Scholar] [CrossRef]
  11. Scalbert, J.; Thibault-Starzyk, F.; Jacquo, R.; Morvan, D.; Meunier, F. Ethanol condensation to butanol at high temperatures over a basic heterogeneous catalyst: How relevant is acetaldehyde self-aldolization? J. Catal. 2014, 311, 28–32. [Google Scholar] [CrossRef]
  12. Makshina, E.; Dusselier, M.; Janssens, W.; Degrève, J.; Jacobs, P.; Sels, B. Review of old chemistry and new catalytic advances in the on-purpose synthesis of butadiene. Chem. Soc. Rev. 2014, 43, 7917–7953. [Google Scholar] [CrossRef]
  13. Jordison, T.; Lira, C.; Miller, D. Condensed-Phase Ethanol Conversion to Higher Alcohols. Ind. Eng. Chem. Res. 2015, 54, 10991–11000. [Google Scholar] [CrossRef]
  14. Shimizu, K. Heterogeneous catalysis for the direct synthesis of chemicals by borrowing hydrogen methodology. Catal. Sci. Technol. 2015, 5, 1412–1427. [Google Scholar] [CrossRef]
  15. Pang, J.; Zheng, M.; He, L.; Li, L.; Pan, X.; Wang, A.; Wang, X.; Zhang, T. Upgrading ethanol to n-butanol over highly dispersed Ni–MgAlO catalysts. J. Catal. 2016, 344, 184–193. [Google Scholar] [CrossRef]
  16. Ogo, S.; Onda, A.; Iwasa, Y.; Hara, K.; Fukuoka, A.; Yanagisawa, K. 1-Butanol synthesis from ethanol over strontium phosphate hydroxyapatite catalysts with various Sr/P ratios. J. Catal. 2012, 296, 24–30. [Google Scholar] [CrossRef]
  17. Cimino, S.; Apuzzo, J.; Lisi, L. MgO Dispersed on Activated Carbon as Water Tolerant Catalyst for the Conversion of Ethanol into Butanol. Appl. Sci. 2019, 9, 1371. [Google Scholar] [CrossRef]
  18. Perrone, O.M.; Lobefaro, F.; Aresta, M.; Nocito, F.; Boscolo, M.; Dibenedetto, A. Butanol synthesis from ethanol over CuMgAl mixed oxides modified with palladium (II) and indium (III). Fuel Process. Tech. 2018, 177, 353–357. [Google Scholar] [CrossRef]
  19. Ungureanu, A.; Dragoi, B.; Chirieac, A.; Ciotonea, C.; Royer, S.; Duprez, D.; Dumitriu, E. Composition-dependent morphostructural properties of Ni-Cu oxide nanoparticles confined within the channels of ordered mesoporous SBA-15 silica. ACS Appl. Mater. Interfaces 2013, 5, 3010–3025. [Google Scholar] [CrossRef]
  20. Riittonen, T.; Toukoniitty, E.; Madnani, D.K.; Leino, A.-R.; Kordas, K.; Szabo, M.; Sapi, A.; Arve, K.; Wärnå, J.; Mikkola, J.-P. One-Pot Liquid-Phase Catalytic Conversion of Ethanol to 1-Butanol over Aluminium Oxide—The Effect of the Active Metal on the Selectivity. Catalysts 2012, 2, 68–84. [Google Scholar] [CrossRef]
  21. Benito, P.; Vaccari, A.; Antonetti, C.; Licursi, D.; Schiarioli, N.; Rodriguez-Castellon, E.; Raspolli Galletti, A.M. Tunable copper-hydrotalcite derived mixed oxides for sustainable ethanol condensation to n-butanol in liquid phase. J. Clean. Prod. 2019, 209, 1614–1623. [Google Scholar] [CrossRef]
  22. Nezam, I.; Peereboom, L.; Miller, D.J. Continuous condensed-phase ethanol conversion to higher alcohols: Experimental results and techno-economic analysis. J. Clean. Prod. 2019, 209, 1365–1375. [Google Scholar] [CrossRef]
  23. Eslava, J.L.; Sun, X.; Gascon, J.; Kapteijn, F.; Rodríguez-Ramos, I. Ruthenium particle size and cesium promotion effects in Fischer-Tropsch synthesis over high-surface-area graphite supported catalysts. Catal. Sci. Technol. 2017, 7, 1235–1244. [Google Scholar] [CrossRef]
  24. Guerrero-Ruiz, A.; Badenes, P.; Rodríguez-Ramos, I. Study of some factors affecting the Ru and Pt dispersions over high surface area graphite-supported catalysts. Appl. Catal. A 1998, 173, 313–321. [Google Scholar] [CrossRef]
  25. Yuvaraj, S.; Fan-Yuan, L.; Tsong-Huei, C.; Chuin-Tih, Y. Thermal Decomposition of Metal Nitrates in Air and Hydrogen Environments. J. Phys. Chem. B 2003, 107, 1044–1047. [Google Scholar] [CrossRef]
  26. Morales, M.V.; Conesa, J.M.; Guerrero-Ruiz, A.; Rodríguez-Ramos, I. Tunable selectivity of Ni catalysts in the hydrogenation reaction of 5-hydroxymethylfurfural in aqueous media: Role of the carbon supports. Carbon 2021, 182, 265–275. [Google Scholar] [CrossRef]
  27. Lv, Y.; Li, J.; Feng, S.; Liu, P.; Hao, F.; Xiong, W.; Luo, H. Multi-walled carbon nanotubes supported nickel nanoparticles doped with magnesia and copper for adiponitrile hydrogenation with high activity and chemoselectivity under mild conditions. Chem. Eng. J. 2018, 346, 203–216. [Google Scholar] [CrossRef]
  28. Patil, P.T.; Liu, D.; Liu, Y.; Chang, J.; Borgna, A. Improving 1,3-butadiene yield by Cs promotion in ethanol conversion. Appl. Catal. A 2017, 543, 67–74. [Google Scholar] [CrossRef]
  29. Jeong, Y.; Kim, I.; Kang, J.; Jeong, H.; Park, J.K.; Park, J.H.; Jung, J. Alcohol-assisted low temperature methanol synthesis from syngas over Cu/ZnO catalysts: Effect of pH value in the co-precipitation step. J. Mol. Catal. A Chem. 2015, 400, 132–138. [Google Scholar] [CrossRef]
  30. Liu, Y.-X.; Wang, H.-H.; Zhao, T.-J.; Zhang, B.; Su, H.; Xue, Z.-H.; Li, X.-H.; Chen, J.-S. Schottky Barrier Induced Coupled Interface of Electron-Rich N-Doped Carbon and Electron-Deficient Cu: In-Built Lewis Acid–Base Pairs for Highly Efficient CO2 Fixation. J. Am. Chem. Soc. 2019, 141, 38–41. [Google Scholar] [CrossRef]
  31. López-Olmos, C.; Morales, M.V.; Guerrero-Ruiz, A.; Ramirez-Barria, C.; Asedegbega-Nieto, E.; Rodríguez-Ramos, I. Continuous Gas-Phase Condensation of Bioethanol to 1-Butanol over Bifunctional Pd/Mg and Pd/Mg–Carbon Catalysts. ChemSusChem 2018, 11, 3502–3511. [Google Scholar] [CrossRef] [PubMed]
  32. Quesada, J.; Faba, L.; Díaz, E.; Ordóñez, S. Tuning the selectivities of Mg-Al mixed oxides for ethanol upgrading reactions through the presence of transition metals. Appl. Catal. A 2018, 559, 167–174. [Google Scholar] [CrossRef]
  33. Almohalla, M.; Gallegos-Suarez, E.; Arcoya, A.; Rodríguez-Ramos, I.; Guerrero-Ruiz, A. Comparative study of bioethanol transformation catalyzed by Ru or Pt nanoparticles supported on KL zeolite. Catal. Sci. Technol. 2016, 6, 521–529. [Google Scholar] [CrossRef]
  34. Patel, A.D.; Telalović, S.; Bitter, J.H.; Worrell, E.; Patel, M.K. Analysis of sustainability metrics and application to the catalytic production of higher alcohols from ethanol. Catal. Today 2015, 239, 56–79. [Google Scholar] [CrossRef]
  35. Bisarya, A.; Kathuria, L.; Kanu Das, K.; Yasmin, E.; Jasra, R.V.; Dholed, S.; Kumar, A. State-of-the-art advances in homogeneous molecular catalysis for the Guerbet upgrading of bio-ethanol to fuel-grade bio-butanol. Chem. Commun. 2025, 61, 2906–2925. [Google Scholar] [CrossRef]
  36. López Olmos, C. Producción Catalítica de Compuestos Químicos a Partir de Materiales Renovables. Ph.D. Thesis, Universidad Nacional de Educación a Distancia, Madrid, Spain, 2019. Available online: https://hdl.handle.net/20.500.14468/17508 (accessed on 31 January 2025).
Catalysts 15 00272 sch001
Scheme 1. Steps in the Guerbet reaction mechanism.
Scheme 1. Steps in the Guerbet reaction mechanism.
Catalysts 15 00272 sch001
Catalysts 15 00272 g001
Figure 1. Hydrogen TPR profiles of the different studied materials.
Figure 1. Hydrogen TPR profiles of the different studied materials.
Catalysts 15 00272 g001
Catalysts 15 00272 g002
Figure 2. TEM images of reduced catalysts (left) and catalysts after reaction (right): (a) 5Cu/HSAG, (b) 5Ni/HSAG, and (c) 4Cu1Ni/HSAG.
Figure 2. TEM images of reduced catalysts (left) and catalysts after reaction (right): (a) 5Cu/HSAG, (b) 5Ni/HSAG, and (c) 4Cu1Ni/HSAG.
Catalysts 15 00272 g002
Catalysts 15 00272 g003
Figure 3. TEM images of reduced catalysts (left) and catalysts after reaction (right): (a) 5Cu-Mg/HSAG, (b) 5Ni-Mg/HSAG, (c) 2.5Cu2.5Ni-Mg/HSAG, (d) 4Cu1Ni-Mg/HSAG, (e) 4.75Cu0.25Ni-Mg/HSAG, (f) 4Cu1Ni-Mg/HSAG*.
Figure 3. TEM images of reduced catalysts (left) and catalysts after reaction (right): (a) 5Cu-Mg/HSAG, (b) 5Ni-Mg/HSAG, (c) 2.5Cu2.5Ni-Mg/HSAG, (d) 4Cu1Ni-Mg/HSAG, (e) 4.75Cu0.25Ni-Mg/HSAG, (f) 4Cu1Ni-Mg/HSAG*.
Catalysts 15 00272 g003
Catalysts 15 00272 g004
Figure 4. EDX mappings of catalysts after reaction at 503 K and 50 bar: (a) 2.5Cu2.5Ni-Mg/HSAG, (b) 4Cu1Ni-Mg/HSAG, (c) 4.75Cu0.25Ni-Mg/HSAG. Cu is represented in red, Ni in green, and Mg in white.
Figure 4. EDX mappings of catalysts after reaction at 503 K and 50 bar: (a) 2.5Cu2.5Ni-Mg/HSAG, (b) 4Cu1Ni-Mg/HSAG, (c) 4.75Cu0.25Ni-Mg/HSAG. Cu is represented in red, Ni in green, and Mg in white.
Catalysts 15 00272 g004
Table 1. SBET values determined for the graphite support and the reduced catalytic materials.
Table 1. SBET values determined for the graphite support and the reduced catalytic materials.
MaterialSBET (m2/g)
HSAG399
Mg/HSAG317
5Cu-Mg/HSAG270
5Ni-Mg/HSAG303
4Cu1Ni-Mg/HSAG243
4Cu1Ni-Mg/HSAG *214
* This catalyst was treated at 723 K in helium before reduction.
Table 2. Acidity distribution of the catalysts.
Table 2. Acidity distribution of the catalysts.
CatalystWeak Acids (%)Medium Acids (%)Strong Acids (%)Areas of NH3 TPD (a.u.)
5Cu/HSAG3531341.3
5Ni/HSAG3126431.3
Mg/HSAG3633291.6
5Cu-Mg/HSAG3431352.6
5Ni-Mg/HSAG2932392.0
4Cu1Ni-Mg/HSAG4031291.9
4Cu1Ni-Mg/HSAG *2930411.7
* This catalyst was treated at 723 K in helium before reduction.
Table 3. Basicity distribution of the catalysts.
Table 3. Basicity distribution of the catalysts.
CatalystWeak Bases (%)Medium Bases (%)Strong Bases (%)Total Uptakes (µmol CO2/g)
Mg/HSAG26551965
Mg/HSAG *23562185
5Cu-Mg/HSAG26621254
5Ni-Mg/HSAG13493886
4Cu1Ni-Mg/HSAG1577870
4Cu1Ni-Mg/HSAG *12701869
* These samples were treated at 723 K in helium before reduction.
Table 4. Catalytic performance after 24 h on reaction at 503 K and 50 bar, conversion and selectivity (WHSV in ethanol was 1.9 h−1), and average particle sizes of the catalysts obtained by TEM.
Table 4. Catalytic performance after 24 h on reaction at 503 K and 50 bar, conversion and selectivity (WHSV in ethanol was 1.9 h−1), and average particle sizes of the catalysts obtained by TEM.
Selectivity (%)
CatalystConv. (%)ButOHAcCOCH41,1-DEEOthersd (nm) ad (nm) b
Cu/HSAG174520031134.711.1
Ni/HSAG2372415251284.98.9
4Cu-1Ni/HSAG2053981213234.27.1
5Cu-Mg/HSAG2626230015367.58.9
5Ni-Mg/HSAG23241215252224.55.5
2.5Cu2.5Ni-Mg/HSAG28321311152275.89.3
4Cu1Ni-Mg/HSAG243922464256.67.1
4.75Cu0.25Ni-Mg/HSAG2831232210326.88.5
4Cu1Ni-Mg/HSAG *204421323278.89.7
ButOH—1-butanol, Ac—acetaldehyde, 1,1-DEE—1,1-diethoxy ethane. d (nm)—TEM average particle size of freshly reduced catalysts (a), and after 24 h in reaction (b). * Catalyst treated at 723 K in helium atmosphere prior in situ reduction.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Rodríguez-Ramos, I.; Lopez-Olmos, C.; Guerrero-Ruiz, A. Butanol Production by Ethanol Condensation: Improvements and Limitations in the Rational Design of Cu-Ni-MgO/Graphite Catalysts. Catalysts 2025, 15, 272. https://doi.org/10.3390/catal15030272

AMA Style

Rodríguez-Ramos I, Lopez-Olmos C, Guerrero-Ruiz A. Butanol Production by Ethanol Condensation: Improvements and Limitations in the Rational Design of Cu-Ni-MgO/Graphite Catalysts. Catalysts. 2025; 15(3):272. https://doi.org/10.3390/catal15030272

Chicago/Turabian Style

Rodríguez-Ramos, Inmaculada, Cristina Lopez-Olmos, and Antonio Guerrero-Ruiz. 2025. "Butanol Production by Ethanol Condensation: Improvements and Limitations in the Rational Design of Cu-Ni-MgO/Graphite Catalysts" Catalysts 15, no. 3: 272. https://doi.org/10.3390/catal15030272

APA Style

Rodríguez-Ramos, I., Lopez-Olmos, C., & Guerrero-Ruiz, A. (2025). Butanol Production by Ethanol Condensation: Improvements and Limitations in the Rational Design of Cu-Ni-MgO/Graphite Catalysts. Catalysts, 15(3), 272. https://doi.org/10.3390/catal15030272

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop